The way you have phrased this makes it hard to understand. This is my guess at what you were trying to ask.

If A, B, and C were all thrown, obviously C is more likely to have the highest number- A can be no higher than 6, B no higher than 7 while C is no lower than 7.

If "A and C" or "B and C" are thrown, the answer is still obviously C.

If "A and B" are thrown, the expected value for A is (1+ 2+ 3+ 4+ 5)/6= 2.5 while the expected value for B is (5+ 6+ 7)/6= 3. B is more likely to have the higher number.